Question

part b - the role of dna polymerase iii

in dna replication in bacteria, the enzyme dna polymerase iii (abbreviated dna pol iii) adds nucleotides to a template strand of dna. but dna pol iii cannot start a new strand from scratch. instead, it adds nucleotides to the 3 end of a pre-existing strand of nucleotides, called a primer, that is hydrogen-bonded to the template strand. each of the four images below shows a strand of template dna (dark blue) with an rna primer (red) to which dna pol iii will add nucleotides.

in which image will adenine (a) be the next nucleotide to be added to the primer?

Explanation:

Identify DNA polymerase III directionality

DNA polymerase III synthesizes DNA in the \(5'
ightarrow3'\) direction. It adds nucleotides to the free \(3'\text{-OH}\) group of the primer. In the diagrams, the primer is red and has a labeled \(\text{OH}\) group indicating its \(3'\) end.

Determine the next template nucleotide

Using Antiparallel Strands, the template strand (dark blue) runs antiparallel to the primer. The \(3'\text{-OH}\) end of the primer aligns with the template nucleotide immediately adjacent to it. DNA pol III will read this next template nucleotide to add the complementary base.

Apply complementary base pairing rules

To add adenine (A) to the growing primer, the template strand must contain the complementary base, thymine (T), at the position immediately adjacent to the primer's \(3'\text{-OH}\) end.

Analyze the four options

• Option 1: The \(3'\text{-OH}\) is on the left. The next template base to the left is C. The complementary base added would be G.
• Option 2: The \(3'\text{-OH}\) is on the left. The next template base to the left is A. The complementary base added would be U/T.
• Option 3: The \(3'\text{-OH}\) is on the left. The next template base to the left is T. The complementary base added would be A.
• Option 4: The \(3'\text{-OH}\) is on the left. The next template base to the left is G. The complementary base added would be C.

Select the correct image

The third image shows the template base T adjacent to the \(3'\text{-OH}\) end of the primer. Therefore, adenine (A) will be the next nucleotide added.

Answer:

• First image (next template base is C)
• Second image (next template base is A)
• Third image (next template base is T) (Correct answer)
• Fourth image (next template base is G)